Radio Broadcasting SystemRadio Broadcasting System

Transmitter Receiver

1 What are the features of AM Radio Broadcasting ?

Different audio sources have different bandwidths “W”

AM radio limits “baseband” bandwidth W to 5kHz

FM radio uses “baseband” bandwidth W to 15kHz

AM Radio Spectrum

Receiver Block DiagramReceiver Block Diagram

RF

Amplifier

IF

Mixer

IF

Amplifier

Envelope

Detector

Audio

Amplifier

Antenna

Speaker

2 Draw a block diagram of a AM receiver and explain its operation

1 Antenna1 Antenna

• The antenna captures electromagnetic energy-its output is a small voltage or current.

• In the frequency domain, the antenna output is

0 frequency

Undesired SignalsDesired Signal

Carrier Frequencyof desired station

• The signal from the antenna is usually very weak. Amplification

is, therefore, necessary.

2 RF2 RF AmplifierAmplifier

• RF stands for radio frequency.

• RF Amplifier amplifies small signals from the antenna to voltage levels appropriate for transistor circuits.

• RF Amplifier also performs a bandpass filter operation on the signal

– Bandpass filter attenuates the frequency components outside the frequency band containing the desired station

RF Amplifier-Frequency DomainRF Amplifier-Frequency Domain

• Frequencies outside the desired frequency bandare attenuated

• Frequency domain representation of the output:

0 frequency

Undesired SignalsDesired Signal

Carrier Frequencyof desired station

• The IF Mixer shifts its input in the frequency domain from the carrier frequency to an intermediate frequency of 455kHz:

3 IF3 IF MixerMixer

0 frequency

Undesired Signals

Desired Signal

455 kHz

• The IF amplifier bandpass filters the output of the IF Mixer, eliminating essentially all of the undesired signals.

4 IF4 IF AmplifierAmplifier

0 frequency

Desired Signal

455 kHz

5 Envelope5 Envelope DetectorDetector• Computes the envelope of its input signal

Input Signal

Output Signal

6 Audio6 Audio AmplifierAmplifier

• Amplifies signal from envelope detector

• Provides power to drive the speaker

• Superhetrodyne receiver is the type used in most modern radio and TV receivers. This receiver was designed by Armstrong

3 Draw a block diagram of a AM superherodyne receiver and explain its operation.

• The first stage is a standard RF amplifier.

• The next stage is the mixer, which accepts two

inputs, the output of the RF amplifier and a

steady sine wave from the local oscillator (LO).

The function of the mixer is to mix the AM signal with a

sine wave to generate a new set of sum and difference

frequencies. It can be shown that the mixer output is

an AM signal with a constant carrier frequency

regardless of the transmitter frequency.

• The next stage is the intermediate-frequency

(IF) amplifier, which provides signal

amplification at a fixed frequency.

• Following the IF amplifier stage is the envelope

detector, which extracts the message signal

from the intermediate radio frequency signal.

• A DC level proportional to the received signal's

strength is extracted from the detector stage

and fed back to the IF amplifiers and sometimes

to the mixer and/or the RF amplifier. This is the

Automatic Gain Control (AGC) level, which allows

relatively constant receiver output for widely

variable received signals.

• Output of the detector is amplified by audio

amplifiers to drive the speaker.

Frequency Conversion

• Mixer performs a frequency

translation/conversion process.

• Consider a 1000-kHz carrier that has been modulated

by a 1-kHz sine wave (AM signal into the mixer), thus

producing side frequencies at 999 kHz and 1001 kHz.

• Suppose that the LO input is a 1455-kHz sine wave. mixer, being a nonlinear device, will generate the following components:

• Frequencies at all of the original inputs: 999 kHz, 1000 kHz, 1001 kHz, and 1455 kHz.

• Sum and difference components of all the original inputs: 1455 kHz ±(999 kHz, 1000 kHz, and 1001 kHz). This means outputs at 2454 kHz, 2455 kHz, 2456 kHz, 454 kHz, 455 kHz, and 456 kHz.

• Harmonics of all the frequency components listed in 1 and 2 and a dc component.

• The IF amplifier has a tuned circuit that only

accepts components near 455 kHz, in this case

454 kHz, 455 kHz, and 456 kHz.

• Since the mixer maintains the same amplitude

proportion that existed with the original AM

signal input at 999 kHz, 1000 kHz, and 1001 kHz,

the signal now passing through the IF amplifiers

is a replica of the original AM signal.

• The only difference is that now its carrier

frequency is 455 kHz. Its envelope is identical to

that of the original AM signal. A frequency

conversion or translation has occurred that has

translated the carrier from 1000 kHz to 455 kHz

• A frequency intermediate to the original carrier

and intelligence frequencies-which led to the

terminology "intermediate frequency amplifier,"

or IF amplifier.

Tuned-Circuit Adjustment

• Now consider the effect of changing the tuned

circuit at the front end of the mixer to accept a

station at 1600 kHz. This means a reduction in

either its inductance or capacitance (usually the

latter) to change its center frequency from 1000

kHz to 1600 kHz.

• The capacitance in the local oscillator's tuned

circuit is simultaneously reduced so that its

frequency of oscillation goes up by 600 kHz.

• The mixer's output still contains a component at

455 kHz (among others), as in the previous case

when we were tuned to a 1000-kHz station. Of

course, the other frequency components at the

output of the mixer are not accepted by the

frequency selective circuits in the IF amplifiers.

• Thus, the key to superheterodyne operation is to

make the LO frequency "track" with the circuit or

circuits that are tuning the incoming radio signal

such that their difference is a constant

frequency (the IF).

• For a 455-kHz IF frequency, the most common

case for broadcast AM receivers, this means the

LO should always be at a frequency 455 kHz

ABOVE the incoming carrier frequency.

• The receiver's "front-end" tuned circuits are

usually made to track together by mechanically

linking (ganging) the capacitors in these circuits

on a common variable rotor assembly.

Image Frequency

• Example: Incoming carrier frequency 1000 kHz,

• Local oscillator = 1000+455=1455 kHz• Consider another carrier at 1910 kHz• If this is passed through the same oscillator, will have a 1910-

1455=455 kHz component• Therefore, both carriers will be passed through IF amplifie• RF filter should be designed to eliminate image signals• The frequency difference between a carrier and its image signal

is:

• RF filter doesn’t have to be selective for adjacent stations, have to be selective for image signals

Therefore,

IFf2

IFRFT fBB 2

Example 2Question: Determine the image frequency for a standard

broadcast band receiver using a 455-kHz IF and tuned to a

station at 620 kHz.

• The first step is to determine the frequency of the LO

• The LO frequency minus the desired station's frequency of 620

kHz should equal the IF of 455 kHz.

Hence,

fLO - 620 kHz = 455 kHz

fLO = 620 kHz + 455 kHz

fLO = 1075 kHz.

Now determine what other frequency, when mixed with 1075

kHz, yields an output component at 455 kHz.

X - 1075 kHz = 455 kHz

X = 1075 kHz + 455 kHz

• Thus, 1530 kHz is the image frequency in this situation.

Automatic Gain Control (AGC)

• The AGC help to maintain a constant output voltage level over a

wide range of RF input signal levels.

• Tuning the receiver would be a nightmare. So as to not miss the

weak stations, you would have the volume control (in the non-

AGC set) turned way up. As you tune into a strong station, you

would probably blow out your speaker while a weak station may

not be audible.

• The received signal from the tuned station is constantly changing

as a result of changing weather and atmospheric conditions. The

AGC allows you to listen to a station without adjusting the volume

control.

• FM radio stations have better quality sound than AM radio stations. Reasons

1 Noise immunity introduced by the non-linear modulation.

2 Bandwidth of FM stations are 15kHz, whereas AM stations are only 5kHz.

• FM receivers can have aerials (antennas) which are half the wavelength of the transmitted carrier (due to the higher frequency of operation). This allows more signal power to be received than the AM.

4 Compare AM radio broadcasting with FM Broadcasting

FM Transmitter

FM signal in Time Domain

FM Radio

• The FM band extends from 88 to 108 MHz. • The maximum information frequency fm is specified as 15 kHz.

(high fidelity)• The minimum bandwidth is to be at least 200 kHz (0.2 MHz).• Therefore, carrier frequencies are separated by 200 kHz.

5 Explain the operation of the FM Superheterodyne Receiver.

• The FM Superheterodyne Receiver has many similarities to that of

the AM Superheterodyne receiver.

• The only apparent differences are the use of the presence of

Limiter-discriminator circuit in place of envelope detector

and

the addition of a de-emphasis network

• RF stage, mixer, local oscillator, and IF amplifiers are basically

similar to those discussed for AM receivers and do not require

further elaboration.

• The universally standard IF frequency for FM is 10.7 MHz, as

compared to 455 kHz for AM.

• A limiter is a circuit whose output is a constant amplitude for all

inputs above a critical value. Its function in a FM receiver is to

remove any unwanted amplitude variations due to noise.

AGC

• In addition to the limiting function also provides AGC action, since

signals from the critical minimum value up to some maximum value

provide a constant input level to the detector.

FM discriminator

• The FM discriminator (detector) extracts the intelligence that has

been modulated onto the carrier via frequency variations.

• It should provide an intelligence signal whose amplitude is dependent

on instantaneous carrier frequency deviation.

• the response is linear in the allowed area of frequency deviation and

that the output amplitude is directly proportional to carrier frequency

deviation.

Pre-emphasis and De-emphasis.

• Despite the fact that FM has superior noise rejection qualities, noise

still interferes with an FM signal. This is particularly true for the

high-frequency components in the modulating signal.

• These high frequencies can at times be larger in amplitude than the

high-frequency content of the modulating signal. This causes a form

of frequency distortion that can make the signal unintelligible.

• To overcome this problem Most FM system use a technique known

as Pre-emphasis and De-emphasis.

• At the transmitter the modulating signal is passing through a

simple network which amplifies the high frequency component more

the low-frequency component.

• The simplest form of such circuit is a simple high pass filter.

• To return the frequency response to its normal level, a de-emphasis

circuit is used at the receiver.

• This is a simple low-pass filter

• The de-emphasis circuit provides a normal frequency response.

• The combined effect of pre-emphasis and de-emphasis is to increase

the high-frequency components during the transmission so that they

will be stronger and not masked by noise.

• This improves the signal-to-noise ratio.

6 Briefly explain the operation of a FM Stereo Broadcasting system

• All new FM broadcast receivers are being built with

provision for receiving stereo, or two-channel

broadcasts.

• The left (L) and right (R) channel signals from the

program material are combined to form two different

signals, one of which is the left-plus-right signal and

one of which is the left-minus-right signal

• An ordinary mono signal consists of the summation of

the two channels, i.e. L + R.

• If a signal containing the difference between the left and

right channels ( L - R) is transmitted then it is possible to

reconstitute the left (L) and right (R) signals.

• Adding (L + R) + (L - R) gives 2L i.e. left signal and

subtracting (L + R) - (L - R) gives 2R, i.e. the right

signal.

• The (L - R) signal is double-sideband suppressed

carrier (DSBSC) modulated about a carrier frequency

of 38 kHz, with the LSB in the 23 to 38 kHz slot and the

USB in the 38 to 53kHz slot.

• The (L + R) signal is placed directly in the 0 to 15 kHz

slot, and a pilot carrier at 19 kHz is added to

synchronize the demodulator at the receiver.

FM Stereo Transmitter

FM Stereo Receiver

• The output from the FM detector is a composite

audio signal containing the frequency-multiplexed (L

+ R) and (L - R) signals and the 19-kHz pilot tone. This

composite signal is applied directly to the input of the

decoding matrix.

• The composite audio signal is also applied to one input

of a phase-error detector circuit, which is part of a

phase locked loop 38-kHz oscillator.

• The output drives the 38-kHz voltage-controlled

oscillator, whose output provides the synchronous

carrier for the demodulator.

• The oscillator output is also frequency divided by 2 (in

a counter circuit) and applied to the other input of the

phase comparator to close the phase locked loop.

• The phase-error signal is also passed to a Schmitt

trigger circuit, which drives an indicator lamp on the

panel that lights when the error signal goes to zero,

indicating the presence of a synchronizing input signal

(the 19-kHz pilot tone).

• The outputs from the 38-kHz oscillator and the filtered

composite audio signals are applied to the balanced

demodulator, whose output is the (L - R) channel.

• The (L + R) and (L - R) signals are passed through a

matrix circuit that separates the L and R signals from

each other.

• These are passed through de-emphasis networks and

low-pass filters to remove unwanted high-frequency

components and are then passed to the two channel

audio amplifiers and speakers.

• On reception of a monaural signal, the pilot-tone indicator circuit goes off, indicating the absence of pilot tone, and closes the switch to disable the (L - R) input to the matrix.

• The (L + R) signal is passed through the matrix to both outputs. An ordinary monaural receiver tuned to a stereo signal would produce only the (L + R) signal, since all frequencies above 15 kHz are removed by filtering, and no demodulator circuitry is present. Thus the stereo signal is compatible with the monaural receivers.